Superalloys are materials, usually nickel based, which have useful properties at temperatures on the order of 1000.degree. F. and above and which are widely used in gas turbine engines. Nickel base superalloys generally consist of a gamma (nickel solid solution) matrix containing a strengthening array of gamma prime phase (Ni.sub.3 Al type) particles. The particle size and distribution can be altered by heat treatment and this also alters the mechanical properties of the alloy.
One important gas turbine engine application for superalloys is turbine and compressor disks. Disks are internal engine components which support and locate the blades in the gas path. In engine operation the disk rotates at speeds of up to about 10,000 rpm (and higher in small engines) and experience temperatures ranging from up to about 1500.degree. F. at the rim to about 500.degree. F. at the center, known as the bore. Disks must have high tensile strength and high creep and stress rupture resistance. In addition, the disk experiences cyclic stresses which can lead to failure if the fatigue properties are inadequate.
While the invention development focused on disk applications, the invention is not so limited.
These property requirements, in the context of the high temperature environment, have led to the use of superalloy disks in virtually all modern turbine engines. Despite the generally acceptable properties exhibited by currently used disks, there is still a need for components which have yet better properties. Improved disk properties can translate into longer disk lives, lighter engines, or permit engine operation at higher rotational speeds.
As noted above, the properties of superalloys can be altered by heat treatment. Many prior art heat treatment developments for disk materials have included heating above the gamma prime solvus temperature. When the gamma prime solvus temperature is exceeded all the gamma prime dissolves leaving nothing to retard grain boundary motion. This leads to rapid grain growth and results in a coarse grain structure, which usually reduces tensile strength and fatigue initiation life but often improves (reduces) the crack growth rate. Conversely, conventional fine grain structures display long times to fatigue crack initiation but then exhibit relatively rapid crack growth rates.
The invention is a heat treatment which provides a fine grain structure that is more resistant to crack initiation and has a lower crack growth rate than prior art treated fine grain material.
Typical of the art are U.S. Pat. Nos. 4,608,094 and 4,624,716. Pending, commonly assigned, U.S. Application No. 733,446 filed 5/10/85 describes a heat treatment for reducing the fatigue susceptibility of gas turbine engine disks, this pending application is currently the subject of a U.S. Patent Office Secrecy Order.